1. Field of the Invention
The present invention relates to improvements in the vehicle frame of a motorcycle.
2. Description of Background Art
A vehicle frame of a motorcycle is disclosed in Japanese Patent Laid-Open No. Sho 62-238180. This vehicle frame, according to FIG. 2 and FIG. 3, includes a frame 2 including a pair of right and left main pipes 202, 202 extended downward toward the rear from a head tube 201 (reference numerals cited herein are from the above-described Japanese Patent Laid-Open publication), right and left lower members 208, 208 extend from the rear end of the main pipes 202, 202, and rear fork pivots 208a, 108a are provided on the right and left lower members 208, 208.
The frame 2 is provided with one down tube 211 extended downward from the head tube 201, two front and rear cross members 203 and 205 between the right and left main pipes 202, 202, and a cross member 209 between the right and left lower members 208, 208.
Furthermore, in the prior art, according to FIGS. 1 and 2 of this publication, an engine is mounted on the right and left main pipes 202, 202, right lower members 208, 208, and the down tube 211; and the front end of the rear fork 10 (corresponding to the rear swing arm) is vertically swingably attached.
Road rebound acting on the rear wheel 6 is transmitted to the right and left main pipes 202, 202 through the rear fork 10, the rear fork pivots 208a, 208a, and the lower members 208, 208.
Generally, the front half section of the frame which includes the head tube 201, the right and left main pipes 202, 202, and the two front and rear cross members 203 and 205 has substantial rigidity. The front half section of the frame forms a main body section of the frame, which receives the weight of the engine suspended thereon, and a fuel tank 4 and a seat 5 mounted thereon, and receives rebound from the road surface during travel.
In the meantime, the rear half section (the rear fork section) of the frame includes the rear fork 10, the lower members 208, 208, the rear fork pivot portions 208a, 208a, and the cross member 209 also has necessary rigidity.
During a turn or banking, a horizontal force acts on the frame 2 of the motorcycle. With the drivability of the motorcycle taken into account, the frame 2 is more desirable to have some degree of flexibility in relation to the lateral load. It is convenient therefore to set, by each vehicle model, the optimum value of the rigidity (connection rigidity) of the portion where the front half section of the frame and the rear half section of the frame are connected.
According to the prior art, therefore, the right and left lower members 208, 208 are jointed for connection to the right and left main pipes 202, 202; to set the connection rigidity, it becomes imperative to change the rigidity of the front half section of the frame and the rigidity of the rear half section of the frame. Therefore, it is not easy to set proper connection rigidity according to the vehicle model.
The present invention, therefore, has as an object the provision of the vehicle frame of a motorcycle that is capable of ensuring the rigidity of the front half section and the rear half section of the frame, and also easily setting the connection rigidity of the rear half section of the frame with respect to the front half section of the frame.
To accomplish the above-described object, according to the present invention, a pair of right and left main frames extend from the head pipe toward the rear; a cross member is provided between the rear ends of the right and left main frames; one center frame extends downward from the cross member; at the lower end of the center frame, an engine mounting lower bracket is jointed; and a swing arm pivot is provided on the center frame or the lower bracket.
The front half section of the frame including the head pipe, right and left main frame, and lower cross member has great rigidity. In the meantime, the rear half section of the frame consisting of the center frame, swing arm pivot, and rear swing arm to be attached to the pivot also has necessary rigidity. It is possible to set a proper connection rigidity of the rear half section of the frame in relation to the front half section of the frame.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.